PHNOM PENH: The US embassy in Phnom Penh said on Friday its defence attaché had been refused full access to Cambodia's largest naval base during an invited visit, just days after Washington expressed concern about China's military activities at the base.

While on a trip to Cambodia on June 1, US Deputy Secretary of State Wendy Sherman raised questions about China's military presence at the Ream Naval Base and asked for clarification about the demolition of US-funded buildings there.

Prime Minister Hun Sen agreed after meeting Sherman to let the US embassy conduct regular visits, with defence attaché Colonel Marcus Ferrara invited to the base on Friday.

"During the brief visit, Cambodian military officials refused to allow the Defence Attaché full access to the Naval Base," the embassy said in the statement.

"When it became clear he would not be granted adequate access, Colonel Ferrara ended the tour and requested Cambodian military officials reschedule the visit with full access at the earliest opportunity," the statement said.

The embassy said routine and frequent visits by US and other foreign military attachés to the base would be an important step towards greater transparency and mutual trust.

Ouk Seyha, commander of the Ream Naval Base, declined to comment to Reuters when asked about the visit.

Chhum Socheat, a spokesman at Cambodia's defence ministry, could not be reached for comment.

In October, Cambodia confirmed it had razed a small US-built facility at the base as part of a planned upgrade, but denied reports that China would be involved in that.

Defence Minister Tea Banh said last week that China will help to modernise and expand Ream, but will not be the only country given access to the facility.

Cambodia has moved closer to China to become one of its most important allies in Southeast Asia, at a time when Washington has been seeking to counter Beijing's growing influence in the region.